The optical sensing and photocatalysis devices described here utilize the interaction between (a) at least one target substance, (b) light at specific wavelength(s), and in the case of photocatalysis (c) a photocatalyst, at or in proximity to the device structures. A device can be used as a gas or liquid sensor by measuring the absorption level of the optical output due to the optical excitation of the target substance. It can also be used as a means of photocatalysis to convert undesirable organic materials such as a toxic gas or a virus into (i) a modified substance whose removal or destruction is more manageable, through the reactions with radicals created through oxidation and reduction of the undesirable materials or (ii) a third substance such as water. The prior art for gas sensing devices and the prior art for photocatalysis devices are described.
Out of several techniques used for gas sensing such as metal-oxide, electrochemical, catalytic beads, a nondispersive infrared (NDIR) sensor is attractive for potentially enhanced sensitivity and the ability to react specifically with the target substance. It is based on the emission of light with a specific wavelength into a target substance and measuring the absorption of the optical power. Since it operates at a specific wavelength where only a particular material has relatively high absorption, an NDIR sensor has the ability to specify the target substance as well. Unlike the cases of other gas sensors, an NDIR sensor has relatively long life time as well as a rapid response time because it is not subject to a chemical reaction. Further, since the measurement is based on photodetection, it generally has higher sensitivity than other types of sensors.
A sensor based on NDIR includes an optical emitter, a detector, and an optical waveguiding device. The absorption occurs when a target substance is present in the optical path between the emitter and the detector where the wavelength of the light matches at least one of the lines in the absorption spectrum of the target substance. As long as the specific wavelength has the absorption peaks dominantly at the target substance, the amount of the target substance can be measured based on the absorption level. In order to achieve a highly sensitive sensor so that the target substance can be detected even at low concentration, the waveguide is preferably designed to be long so that the amount of contact between the target substance and the waveguide is increased.
In prior art NDIR sensors, waveguides were used between the emitter and detector to guide emitted light to the detector while creating the contact area for the target substance to be measured. Wong (U.S. Pat. Nos. 5,444,249, 5,747,808, 5,834,777) describes sensors that are based on the integration of waveguides with a light source and a photodiode, and have a gas inlet. The advantages of using waveguide devices include small form factor and low cost. These waveguides include an under-cladding layer, a core layer, and at least one over-cladding layer on a substrate that is typically silicon. Since the waveguides in most cases are fabricated utilizing lithographic processes with a silicon substrate, the size of the waveguide chips is limited by the size of the substrate wafer and the number of chips patterned on each wafer, and this size is typically on the order of a square millimeter to a square centimeter. The small form factor in the prior art meant that a small area was available for the target substance to be in contact with the waveguide and to be measured.
In other prior art, improved waveguide structures were described for the realization of higher sensitivity devices. O'Keefe (U.S. Pat. No. 6,694,067) describes a waveguide with a pair of gratings that act as a cavity so that light is confined within the cavity for effectively longer propagation before reaching the detector. Janz (U.S. Pat. No. 7,778,499) describes a waveguide where the over-cladding layer was eliminated in order to enhance the relative amount of its spatial mode field in the air where the target substance is present. Digonnet (U.S. Pat. No. 8,427,651) describes a hollow core waveguide for the effective light confinement within air. Yamashita (U.S. Pat. No. 8,542,957), Tao (U.S. Ser. No. 10/215,692) and Gylfason (U.S. Ser. No. 10/598,590) describe waveguide structures with partial substrate removal so that sensing of the target substance can be achieved both from the top of the over-cladding and the bottom of the under-cladding. Although improved performance is expected in these prior art cases, the improvement is limited because it uses only a single waveguide layer.
Juni (U.S. Pat. No. 8,135,246) taught a sensor based on stacking a plurality of single-layer waveguide devices with a plurality of emitters and detectors. With the use of angular offset from the adjacent layer of a waveguide chip, the effective contact to the target substance can be achieved. However, this is an expensive method as the cost increases with the number of single-layer waveguide devices.
The present embodiments include the formation of photocatalysis devices. A photocatalysis device is a well-known device that absorbs a specific-wavelength light whose energy level is comparable to the bandgap of the device material, and generates an electron-hole pair that leads to a chemical reaction. For example, titanium dioxide absorbs ultraviolet (UV) light to generate an electron-hole pair which, in the presence of water and oxygen, generates radicals that react with and mineralize undesirable organic compounds, such as the lipid membranes that envelope and protect viruses such as Coronavirus Disease 2019 (COVID-19), allowing to pry apart the membranes and destroy the cells. In order for a photocatalytic device to be effective, the surface area where the photocatalyst (photocatalysis material) is exposed to said specific-wavelength light needs to be large.
Prior art by Kurihara (U.S. Pat. No. 7,683,005), Morito (U.S. Pat. No. 9,061,086), Lee (U.S. Pat. No. 9,744,257), Ozaki (U.S. Ser. No. 10/201,809), Kitazaki (U.S. Ser. No. 10/434,505) and Leung (U.S. Ser. No. 10/434,505) describe porous materials based on honeycomb structures, particles and fibers. The porous materials increase the effective surface area, however it works on a single surface so that the working area is limited by the area illuminated by said specific-wavelength light.
Mizuno (U.S. Pat. No. 6,324,329) and Li (U.S. Pat. No. 7,625,835) proposed waveguide structures where the outermost (over-cladding) layer comprises a photocatalytic material in order to create a photocatalytic effect. Said specific-wavelength light is guided inside their guiding layers as opposed to illuminating the surface of the photocatalytic material. In these cases, the illumination is not spatially limited. However, the photocatalytic reaction can only occur at one side of the surface as the waveguide structure is exposed to the target material only on one side. Erickson (U.S. Ser. No. 10/604,733) proposed a photocatalyst design with a plurality of waveguides with spaced relationship within the enclosure. In this case again, only the outermost layer of each waveguide comprises a photocatalytic material so that it reacts with the material within the same enclosure. This prior art can expect the efficiency increased with the plurality of waveguides. However, the spaced relationship within the enclosure limits the effective device size.